While trying to route my new transducer cable, I encountered the vented loop from the heat exchanger before it enters the waterlift muffler. I had a choice, undo the two 3/4" hoses, or undo the one ľ" hose.

I decided that rather than messing with two hose clamps, one would be easier - that was a mistake. After I removed the clamp, I could not get the hose to let go of the barb. A little more pressure, and snap, off came the hose with the barb still inside.

So off to WM to get a new one. No joy at the closest WM, so 55 min drive in the other direction where they had 5 in stock. The new vented loop is one with the duckbill valve, and the old one had a barb with a hose.
The original looked more, or less, like this (except it was old - 1987 black plastic);

Here is a pic of the new one;

Because the loop is mounted HORIZONTALLY (not vertically) under the cockpit floor, I was unsure if this new fangled style would work. Fortunately, I found a brass barb fitting that fit, so I bought that too.

I put a little pipe dope on the threads, installed the brass barb, and installed the new vented loop in the old location - after correctly routing the transducer cable.

So, the question for the collective; would the new-fangled style have worked?

...and that's why I removed the duckbill and modified it to match the original design. I can see where the vent tube eventually goes vertical, and it has worked this way since 1987. There is no way that the duckbill could be mounted vertically unless I change a bunch of 3/4" exhaust hose.

The duck bill is just a check valve. It's orientation isn't important; only the direction of flow (and that it's kept clean so that it doesn't stick shut). If the loop is the highest point on the line, and above the heeled waterline, the vacuum isn't going to know whether the plumbing is laying on its side or upright. If the hose starts to siphon seawater into the engine, the valve will break the vacuum.
If you aren't confident in the duckbill, or you won't be able to access it to check it every now and then, you can just get a 1/4" barb with a female threaded end that fits where the cap threads on, and put the 1/4" hose back on. I assume it exits through the cockpit coaming or someplace. Then you can leave the duckbill out.

The duck bill is just a check valve. It's orientation isn't important; only the direction of flow (and that it's kept clean so that it doesn't stick shut). If the loop is the highest point on the line, and above the heeled waterline, the vacuum isn't going to know whether the plumbing is laying on its side or upright. If the hose starts to siphon seawater into the engine, the valve will break the vacuum.
If you aren't confident in the duckbill, or you won't be able to access it to check it every now and then, you can just get a 1/4" barb with a female threaded end that fits where the cap threads on, and put the 1/4" hose back on. I assume it exits through the cockpit coaming or someplace. Then you can leave the duckbill out.

Thanks!

That confirms what I believed, and did.

The issue for me is that I believe that the duckbill is designed to be installed at the absolute peak of the loop when the vented loop is vertical. Turning the vented loop on it's side effectively moves the duckbill to a point slightly less (1 or 2 mm) than the peak, because of the difference in the circumferences of the vent and the loop. My thought is that this may have prevented the vent from opening when it should.

...and that's why I removed the duckbill and modified it to match the original design. I can see where the vent tube eventually goes vertical, and it has worked this way since 1987. There is no way that the duckbill could be mounted vertically unless I change a bunch of 3/4" exhaust hose.

I guess that I made the right decision.

Just be sure your "exit" hose does not have any dips in it that can collect and hold water. With a bad impeller even a small "trap" can cause a siphon.

For engine vacuum breakers I prefer the ones made by the Scot Pump Co. as I find they are far more trouble free than a duck bill....

You can buy just the vacuum breaker:
Or the whole thing:
I tend to just make custom ones:

This was Ericson's idea for a siphon break and it sprayed salt water all over the engine when the brazed copper tube failed. It was also mounted under the cockpit:

A few dollars and 30 minutes later a new one that is significantly more robust replaced it.

My thought is that this may have prevented the vent from opening when it should.

Nah. You're talking a fraction of an inch. If you're worried about the joker valve malfunctioning, just use a small open tube vented outside of the boat.
Recall that the siphon break works because it takes less force to suck air through it than it does to lift a hose full of water out of the sea. I can't see how moving it a half a hose diameter below the peak of the hose ID is going to make drawing air harder then lifting all that water.

Maine sail: How do those vented loops do with the head? Any smell? I'm just cautious because my head hoses are exposed inside the main cabin, nothing I can do about that. I have no vented loops but would like to add them for safety and those look great.

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